1. Field of The Invention.
This invention relates to heat pipes operating as a closed evaporating-condensing cycle for transporting thermal energy More particularly, this invention refers to a modulated heat pipe system which is used to improve the dehumidification of an air conditioning system and/or to provide an efficient heat exchanger which can operate in varying temperature ranges.
2. Description of The Prior Art
Heat pipes can be defined as devices employing closed evaporating-condensing cycles for transporting heat from a locale of heat generation to a locale of heat reception. They may or may not use a capillary structure or wick to facilitate return of the condensate and may be in any shape or geometry. Heat pipes have been recognized for several years as very effective heat transport devices. See U.S. Pat. Nos. 3,756,903 and 4,182,409. They will transport large amounts of heat with small temperature gradients independent of gravity effects, which makes these devices suitable for applications in space.
Thermal control systems using heat pipes or thermal switches have been devised to dissipate heat from equipment such as a spacecraft or an airplane. Heat transfer through the heat pipes has been made variable by means of variable conductance heat pipes (a heat pipe containing non-condensable gas with the heat flow modulated by varying the gas-occupied volume of the condenser or evaporator), thermal switch (a device providing a region of variable thermal conductance which is temperature actuated; i.e., with bimetals or expanding fluids) or thermocouple actuators for positioning louvers to cover the radiating surfaces. These systems have been well-developed in the low-temperature, gravity-free environments such as space, but have disadvantages such as degradation due to vibrational loads, wear or limited heat flux control.
Space vehicles have employed heat pipes for a number of years as described in U.S. Pat. Nos. 3,548,930; 4,162,701 and 4,687,048. The primary motivation in this system is a means for moving thermal energy in a system lacking the pull of gravity. A wick is used in the heat pipes to return the working fluid from the condenser to the evaporator. In addition, reservoirs such as shown in U.S. Pat. No. 3,548,930 have been used to store heat for use when the space vehicle is not exposed to sunlight. Generally, prior art heat pipes have been designed to operate in one temperature range with no modulation of the heat transfer rate.
Heretofore, heat pipes have not found broad commercial use because of the constant temperature requirement and inability to modulate the rate of heat transfer.
Heat pipes have recently been integrated with air conditioning equipment to modify the dehumidification capacity of the cooling system. This offers advantages over other dehumidification technologies in that heat pipes are passive, that is, they require no mechanical parts, and do not require an external energy source. In these systems, excess heat is extracted by the heat pipe evaporator from the return air stream prior to passing over the cooling coil. The heat extracted by the evaporator is moved via working fluid through the heat pipe passage to the condenser portion located downstream from the cooling coil. The air, which has been dehumidified and over-cooled upon leaving the cooling coil, is re-heated to a comfortable temperature using the waste heat extracted from the return air. These systems control the heat pipe using a fully "on" or fully "off" scheme. During the fully "off" mode, tilt mechanisms are used to isolate the working fluid in a condensed form using gravity during conditions when the sensible heat capacity is maximized. The heat pipes are "tilted" such that the evaporator is placed in the heat source allowing the working fluid to extract heat, vaporize and travel to the condenser region. The condenser is placed by the tilt mechanism in the heat sink and releases the heat from the working fluid thereby condensing and traveling, via gravity, down the walls of the heat pipe to the evaporator region.
Instead of using a fully on or off scheme, a modulated system for widespread commercial use is needed. A practical heat pipe system employing a modulated system for regulating heat transfer could response to the soaring energy demands in many regions of the world.